Many electronic circuits have a need for a regulated voltage. FIG. 1 illustrates an example of a conventional difference amplifier 100 that may be used to provide a regulated voltage. The difference amplifier 100 of FIG. 1 regulates a voltage (Vout) by using voltage feedback in a loop having a differential pair of transistors. The differential pair compares a reference bandgap voltage with a pre-determined fraction of the output voltage and produces a drive-signal based on the comparison.
The voltage at the input of transistor Q1 is some fraction of the voltage, Vout, based on the relative sizes of the voltage divider resistors R1, R2, R3, and R4. The difference amplifier 100 keeps the voltage, Vout, regulated by forcing the voltage at the input of transistor Q1 to be equal to the reference voltage Vref. That is, Vout is regulated because any difference between the voltage at the base of transistor Q1 and the voltage at the base of transistor Q2 is forced to zero. By appropriate sizing of voltage divider resistors R1–R4, a suitable voltage Vout may be maintained.
While the conventional difference amplifier illustrated in FIG. 1, as well as other conventional difference amplifiers, are well-suited for a number of applications, they have several limitations.
First, such conventional difference amplifies are often designed to operate over a relatively limited range in output current. However, for some applications a wider range in output current is required, or at least desired. Second, such circuits are often designed to output a single fixed output voltage. However, for some applications it is required, or at least desired, to have a circuit that is able to accurately output a variety of output voltages.
If a difference amplifier such as the one illustrated in FIG. 1 is attempted to be used over too wide a range in output current, the circuit exhibits errors in output voltage as the output current varies. For example, the circuit may only hold to about 85 to 100 milli-volts with a 5 volt output over an output current range from 100 microamperes to 150 milli-amperes. A further limitation of the conventional circuit 100 illustrated in FIG. 1 is that the power supply rejection with the input voltage going from 6 volts to 60 volts is approximately 20 to 30 milli-volts.
Thus, a need exists for a voltage regulation circuit. A further need exists for a voltage regulation circuit that provides good performance over a wide range in output current. A further need exists for a voltage regulation circuit that is able to accurately output a variety of regulated voltages. A still further need exists for a voltage regulation circuit that has a good power supply rejection ratio over a wide range of input supply voltages. A still further need exists for a voltage regulation circuit that is compatible with and can be fabricated economically with existing semiconductor fabrication techniques.